1. Field Of The Invention
The present invention relates to a sulfonic acid group-containing polyurethane and a photosensitive resin composition containing the same. More particularly, the present invention is concerned with a sulfonic acid group-containing polyurethane produced from a sulfonic acid group-containing polyamide diol and/or a sulfonic acid group containing polyester diol, a dihydroxyhydrocarbon having a specific molecular weight and a diisocyanate. Further, the present invention is concerned with a photosensitive resin composition comprising such a sulfonic acid group-containing polyurethane as a base polymer, an ethylenically unsaturated compound and a photopolymerization initiator.
The sulfonic acid group-containing polyurethane of the present invention has not only high hydrophilicity but also excellent mechanical strength, excellent heat resistance, low permanent tensile set and low hardness, which are required for the base polymer for a photosensitive resin composition to be used for producing a flexographic printing plate, and has also excellent compatibility with an ethylenically unsaturated compound. The photosensitive resin composition containing such a sulfonic acid group-containing polyurethane has excellent photosensitivity and, in addition, has excellent properties such that, after the photosensitive resin composition has been exposed to actinic rays through an image-bearing transparency, the development of the exposed composition can be performed with water or an aqueous alkali solution optionally containing a surfactant, thereby producing a flexographic printing plate which has excellent printing resistance, ink resistance and mechanical properties
2. Discussion Of Related Art
Various types of polymers are known as a base polymer of a photosensitive resin composition which is used to produce a printing plate. In particular, polyamides are widely used as such a base polymer. Conventionally, an alcohol-soluble polyamide copolymer was mainly used. However, the production of a printing plate using this alcohol-soluble polyamide copolymer is inevitably accompanied by drawbacks, such as limitations in handling and operation attributed to the use of an alcohol and the necessity of an anti-fire developer apparatus. Therefore, currently, as polyamides for use in preparing a photosensitive resin composition, use is made of those which can provide a photosensitive resin composition which, after being exposed to actinic rays, can be developed with water or an aqueous alkali solution.
Representative examples of such polyamides include a sulfonic acid group-containing polyamide as disclosed in Japanese Patent Application Laid-Open Specification Nos. 48-68302/1973 and 48-72250/1973, a polyamide containing a basic nitrogen or an ammonium salt-forming nitrogen as disclosed in Japanese Patent Application Laid-Open Specification Nos. 50-7605/1975 and 50-78401/1975 and a polyamide having an ether bond as disclosed in Japanese Patent Application Laid-Open Specification No. 48-42049/1973 and Japanese Patent Application Publication Specification No. 49-43565/1974.
The printing plates produced from a photosensitive resin composition containing such a polyamide, however, has poor resistance to flexographic printing ink (in which water or an alcohol is used as a solvent).
A thermoplastic elastomer is also used as a base polymer for a photosensitive resin composition which is useful in producing a flexographic printing plate. As such thermoplastic elastomers, use is currently made of those which, after being exposed to actinic rays, can be developed only with organic solvents, such as ketones and chlorinated hydrocarbons. The use of such organic solvents are likely to cause a pollution problem. Therefore, efforts are made in the art to develop a photosensitive resin composition comprising a thermoplastic elastomer, which composition, after being exposed to actinic rays, can be developed with water or an aqueous solution. Examples of such photosensitive resin compositions are disclosed in for example, Japanese Patent Application Publication Specification Nos. 59-29849/1984 and 58-33884/1983. From any of the proposed photosensitive resin compositions, however, only printing plates having poor ink resistance can be obtained, which limits their application in industry.
Further, a polyamide copolymer comprising a polyamide as a main chain and, incorporated therein, a polyether segment, was proposed as a base polymer for a photosensitive resin composition which can give a flexible, tough flexographic printing plate by exposure to actinic rays and development with water or an aqueous solution. Representative examples of such copolymerized polyamides include a polyamide copolymer as produced from a polyoxyethylene having terminal amino or carboxyl groups and an aliphatic dicarboxylic acid or an aliphatic diamine (see Japanese Patent application Publication Specification Nos. 57-18173/1982 and 57-4891/1982), and a polyether-ester-amide as produced from a lactam or a nylon salt, a polyoxyalkylene glycol and a dicarboxylic acid (see Japanese Patent Application Laid-Open Specification Nos. 58-117537/1983 and 60-14232/1985). Still further, a polyether-ester-amide as produced from a sulfonic acid group-containing polyamide, a polyoxyalkylene glycol and a dicarboxylic acid (see Japanese Patent Application Laid-Open Specification Nos. 60-8322/1985 and 61-162524/1986) and a polyether-ester-amide as produced from a polyamide having a basic nitrogen, a polyoxyalkylene glycol and a dicarboxylic acid (Japanese Patent Application Laid-Open Specification No. 59-221327/1984 and Japanese Patent Application Publication Specification No. 2-372/1990) were proposed as a base polymer for a photosensitive resin composition which is excellent in developability with water or an aqueous solution after exposure to actinic rays. However, from such polyamide copolymers, only a flexographic printing plate having poor resistance to flexographic printing ink is obtained.
Known as a base polymer for a synthetic leather, a coating composition and an adhesive composition is a polyether-ester-urethane as produced from a sulfonic acid group-containing polyester diol, a polyoxyalkylene glycol and a diisocyanate (see Japanese Patent Application Laid-Open Specification Nos. 59-6210/1984 and 59- 24/1984). When this polyether-ester-urethane is used as a base polymer for a photosensitive resin composition and the composition is exposed to actinic rays and developed, the resultant printing plate has poor resistance to flexographic printing ink.